1. Field of the Invention
The present invention relates to a display device in which an element having a structure including an anode, a cathode, and a thin film which emits light by electroluminescence (hereinafter, referred to as “EL”) interposed between the anode and the cathode is provided over a substrate.
2. Description of the Related Art
An EL element is an element which obtains light emission by forming a thin film containing an organic compound or an inorganic compound, or a crystal between a cathode and an anode and feeding current between the cathode and the anode. In recent years, especially an EL element in which a thin film containing an organic compound as its main constituent (hereinafter, referred to as a light-emitting layer) is placed between a cathode and an anode, i.e., an organic EL element, has been actively developed.
An organic EL element is expected to be applied in various fields, and is considered to be applied not only to a lighting apparatus but also to a display or the like used for a mobile phone or a personal computer. The organic EL element is manufactured by interposing a material which emits light by feeding current between a pair of electrodes. The organic EL element emits light by itself, and thus does not need a light source such as a backlight, differently from a liquid crystal. Also, the element itself is extremely thin. Therefore, the organic EL element has a great advantage in manufacturing a thin and lightweight display.
For example, as such a display, a display device is proposed, in which an organic EL element using metal having high reflexivity for a cathode and a transparent electrode for an anode is provided over a substrate. By using the metal having high reflexivity for the cathode, emission luminance from a light-emitting layer can be enhanced; however, on the other hand, there is a problem that an outside image is reflected when external light is reflected on the metal surface, which causes deterioration of image display characteristics, for example a viewer has difficulty in seeing a display image and brightness of the display image gets dark. In order to solve the problem, there is a proposed method of suppressing reflection of an outside image (or outside light) by providing antireflection means. For example, a display device (Patent Document 1: Japanese Published Patent Application No. 2005-100789) provided with antireflection means including a quarter wave plate, a reflective polarizing plate, and an absorptive polarizing plate, or a display device (Patent Document 2: Japanese Published Patent Application No. H11-45058) provided with antireflection means including a wavelength correcting plate, a plane linear polarization beam splitter, and a polarizing plate is disclosed.
However, in the display device provided with antireflection means including a quarter wave plate, a reflective polarizing plate, and an absorptive polarizing plate, or the display device provided with antireflection means including a wavelength correcting plate, a plane linear polarization beam splitter, and a polarizing plate, there are possibilities not only that reflection of an outside image when external light is reflected on a cathode surface cannot be suppressed, but also that part of external light which enters the display device does not pass through the reflective polarizing plate or the plane linear polarization beam splitter and is reflected on the surface, and accordingly, the reflection of the outside image cannot be eliminated completely.